1. Field of the Invention
The present invention relates generally to a carboxylic acid production process in which a concentration step is employed before and/or after a product isolation step. More specifically, the present invention concerns a process for the production of a terephthalic acid product where oxidation byproducts exit a product isolation step with isolated solids comprising terephthalic acid at a rate of at least about 15 percent of the net make rates of the oxidation byproducts in the production process.
2. Description of the Prior Art
In conventional terephthalic acid (TPA) production processes, para-xylene undergoes oxidation. In such processes, oxidation byproducts are produced along with the formation of TPA. Typically, such oxidation byproducts include the oxidation intermediates and side reaction products formed in the oxidation of para-xylene, as well as any impurities originating from the raw materials. Some of these byproducts are detrimental to the use of TPA in various production processes, such as for the production of polyethylene terephthalate (PET), dimethyl terephthalate (DMT), or cyclohexane dimethanol (CHDM). For example, some oxidation byproducts can act as chain terminators in a PET polymerization process, thus decreasing the molecular weight of the PET product. Accordingly, conventional TPA production processes remove substantially all of the oxidation byproducts produced therein.
It is known in the art to employ a purge process to remove oxidation byproducts from TPA production processes. A purge process typically involves separating a portion of a mother liquor, generated from the separation of liquid from the product stream, to form a purge feed stream. The purge feed stream generally constitutes in the range of from 5 to 40 percent of the total mother liquor, but can be up to 100 percent of the mother liquor. In a typical conventional purge process, the purge feed stream contains acetic acid, catalyst, water, oxidation byproducts, and minor amounts of terephthalic acid. The purge feed stream in conventional processes is usually resolved into a catalyst rich stream and an oxidation byproduct rich stream. The catalyst rich stream is typically recycled to the oxidizer, whereas the oxidation byproduct rich stream is usually routed out of the TPA production process for waste treatment or destruction.
Despite the advantages gained from a purge process, concerns arise regarding employing a purge in a TPA production process due to the added cost of producing the TPA product. Additionally, the removal of oxidation byproducts decreases product yield, further increasing the cost of the final product. Accordingly, there is a need for a TPA production process wherein at least a portion of the oxidation byproducts exit the production process with the TPA product and/or are combined with the TPA product downstream of the process.